The field of the disclosure relates generally to the storage and delivery of cryogenic fuels, and more specifically, to a system for maintaining a constant pressure in a cryogenic fuel tank.
The cost of petroleum-based liquid fuels has been escalating because of continuing world demand, and demand will likely continue to increase because of substantial economic growth in countries such as China and India. These countries have relatively large populations, spurring rapid expansion of ground and air transportation.
The increasing cost of petroleum-based products is due, at least in part, to a dependence on importing energy. This has resulted in numerous efforts to develop alternative energy sources to reduce costs and achieve energy independence. For example, recent advances in shale gas fracking have led to large-scale recovery of natural gas from shale rock. This abundance of natural gas has led to a decrease in the cost of natural gas, while the cost of petroleum-based fuels continues to rise. Accordingly, at least some known vehicle fuel systems utilize natural gas instead of petroleum-based fuels.
For example, liquefied natural gas may be used as a fuel for an aircraft. The liquefied natural gas may be stored in light-weight cryogenic fuel storage tanks onboard the aircraft. However, changes in altitude of the aircraft may result in pressure changes in the fuel storage tanks, causing at least a portion of the total liquefied natural gas to boil off. Accordingly, to avoid boil off, it is desirable to maintain a substantially constant pressure within the fuel storage tanks. However, as liquefied natural gas is essentially pure methane, the substantially constant pressure should not be maintained by venting liquefied natural gas into the atmosphere.